An electric motor generally has an armature and its winding which combines with a stator to produce torque. A generator also has an armature and its winding which combines with a stator to produce emf (electromotive force). The armature winding is formed by various methods, but a main method used is a drum winding. Drum windings include two kinds, one called a lap (or multiple) winding or a parallel winding and the other a wave winding or a series winding. The former is generally used in an electric machine for large capacity (or current) and the latter is used in a machine for medium or small capacity or comparatively high voltage.
The machines having an armature with its winding based on the above-mentioned method may have such problems as biasing of the magnetic neutral phase and cross-magnetism effect, which leads to commutating difficulty, having the effect of a decrease in emf produced in a generator or a decrease of torque produced in a motor. So extra compensating windings and intermediate poles with windings are needed to be additionally provided to the ordinary windings, so as to offset the emf that is induced by flux generated by reaction of the armature. But, the addition of the compensating windings and the intermediate pole, and its winding, can enlarge the dimension of a motor and makes its structure more complicated.